He was nearing the end of a long skywatching session Sunday when his telescope's camera recorded what looked like a fresh dark collision scar on Jupiter's cloud tops. It was moving too slowly to be a Jovian moon's shadow, and it wasn't there in observations he'd made a few nights earlier. You can read his full account of the observation at his website.

The mark was similar to marks left after comet Shoemaker-Levy 9's buckshot-like collision with Jupiter in 1994. The comet had broken into 21 fragments and would pepper the giant planet between July 16 and July 22.